In the galvanizing of steel, adherent zinc coatings depend upon the ferrous base metal strip entering the molten zinc based bath with the strip surface essentially free of oxide and dirt. Accordingly, after the strip is heated and cleaned in the galvanizing line furnace sections, a protective or non-oxidizing atmosphere is maintained about the strip prior to its entry into the zinc bath.
This protective or non-oxidizing atmosphere may have insufficient activity of oxygen necessary to prevent the formation of zinc vapor. Consequently, zinc vapor will migrate up into the entry section, cooling section, and various furnace sections of the galvanizing line. Generally the zinc vapor condenses in the entry and cooling sections, effecting a phase change into solid or liquid metallic zinc or zinc oxide, and accumulates on the various elements of the entry and cooling sections, and falls from the elements onto and alloys with the clean ferrous base metal strip. It is theorized that as zinc droplets fall on the strip, the outer surface of each droplet oxidizes forming a zinc droplet surrounded by a Zn oxide film. Upon impact of the droplet on the strip, the droplet flattens out and the zinc metal alloys with the ferrous strip, while the zinc oxide forms into a flake. The zinc oxide flake does not alloy with the ferrous strip nor does it strongly adhere to the iron-zinc alloy layer. Consequently, during immersion in the zinc coating metal, the spots created by the droplets are not adhered to by the molten zinc and after exiting the metering device they appear as non-uniform, uncoated portions on the strip. These coating defects are undesirable.
U.S. Pat. No. 4,369,211 to Nitto et al recognizes the problem of zinc vapor formation in a coating chamber, rather than the snout chamber. Specifically, Nitto et al maintain a controlled atmosphere of about 50-1000 ppm oxygen in the coating chamber which is sufficient to prevent zinc vapor formation.
Belgium Pat. No. 887,940 to Heurtey recognizes the problem of zinc vapor formation in the snout section. In particular, a sweep gas is employed, not to prevent zinc vapor formation, but to sweep over the hot dip zinc based bath surface and become loaded with zinc vapor. The loaded sweep gas is evacuated from the snout and undergoes condensation to recover the zinc based coating.
Neither Nitto et al nor Heurtey comprises an economical procedure for adequately suppressing zinc vapor formation in the snout. In particular, 50 ppm molecular oxygen described by Nitto et al may result in a thin oxide film on the clean ferrous base metal strip, which, if not dissolved in the coating pot by the zinc, can result in poor adherence of the zinc coating to the ferrous strip. With respect to Heurtey, employing a sweep gas and treating it to recover zinc or zinc oxide is especially costly, requiring additional personnel and additional maintenance.
Accordingly, there exists a need for a process to suppress zinc vapor formation which does not require additional costly equipment and maintenance, nor yield coating defects because of poor adherence.